WVM, I think Len is on to something. Even though mine is a small flat pan rig, I made a change today. I was firing ever 7 minutes and I could get my stainless stack to glow red the first foot or so. Daryl suggested I put small pieces of brick in my flue area at the back of the pan. I'm boiling now. I've increased my firing to at least 9 minutes. No red stack. Haven't check GPH but full pan boil so it should be the same. What it comes do to is that I'm keeping more heat in the rig longer. I think every rig is just a little different. When adding the AOF we need to find that optimal set up.